It is known that fiber-reinforced moldings can be produced from a number of different synthetic polymers. Glass fibers have proved to be a particularly advantageous reinforcing material. For example, it is known that resins containing unsaturated polyesters (referred to hereinafter as "UP-resins") can be processed with glass fibers to form storable semi-finished products (resin mats, prepregs; molding compositions) which can be formed by means of heated presses and hardened to form moldings combining high strength with rigidity. This process is, however, attended by several disadvantages. Thus, during the free radical initiated polymerization reaction, the monomers containing vinyl groups (for example styrene) normally used as solvents for the UP-resins lead to a very high crosslink density which in turn makes the molding extremely brittle and reduces its notched impact strength. In addition, the presence of solvents complicates processing since elaborate measures must be taken for extracting the solvent vapors and explosion-proof processing installations are generally necessary.
It is also known that polyurethane elastomers can be reinforced with fibers. Thus, glass-fiber-reinforced polyurethane moldings can be produced, for example, by the injection-molding or reaction injection-molding process. The mechanical properties of these moldings, and paricularly their stiffness in flexure, can only be improved to a limited extend because the length of the fibers used may not exceed about 1 to 6 mm for processing reasons. It is particularly disadvantageous that, due to the limited length of the fibers, the coefficient of thermal expansion of the reinforced polyurethane elastomers is still several times higher than that of steel.
German Pat. No. 968,566 describes a process for the production of high molecular weight crosslinked plastics in which an intermediate product is initially prepared from a polyester containing hydroxyl groups, a glycol and a subequivalent amount of diisocyanate. The intermediate is subsequently converted into storable semi-finished products by reaction with an excess of a diisocyanate containing uretdione groups. Finally, these semi-finished products may be subjected to plastic forming and hardened by the action of heat to form elastic moldings.
These polyurethane elastomers have a predominantly linear structure. Although they combine high elasticity with toughness, they are not hard and stiff enough for numerous applications. The use of fibrous reinforcing material is not described in the above-noted German patent. Although glass fibers having a length of greater than 6 mm, could in principle, be incorporated into the above-mentioned intermediate product, the high viscosity of the intermediate product would necessitate mixing rolls or kneaders of the type normally used in the processing of rubber for mixing in the fibers and the uretdione diisocyanate required for crosslinking. If the fibers were incorporated in this way, however, such intense shear forces would occur that the individual fibers would be reduced to fractions of their original length. It would, thus, not be possible to fully obtain the required effects of reinforcement on the one hand and reduction of the coefficient of thermal expansion on the other.
The object of the present invention is to provide new solvent-free storable molding compositions which may be hardened by molding at elevated temperature to form moldings combining extreme stiffness with high impact strength and high dimensional stability. This object is achieved by the molding compositions provided by the present invention.